Stabilized alumina pebbles



Patented Mar. 10, 1953 STABILIZED ALUMINA PEBBLES Sam P. Robinson,Bartlesville, kla., assignor to Phillips Petroleum Company, acorporation of Delaware No Drawing. Application October 8, 1948, SerialNo. 53,589

9 Claims.

' The invention relates to the manufacture of stabilized alpha aluminapebbles for use in pebble heaters and in other heat exchangeapplications. Specific aspects of the invention pertain to aluminapebbles having high breakage resistance under severe conditions ofcyclic thermal and mechanical shock and high resistance to attrition inmoving bed cyclic heat transfer duty, and to a method of manufacturingsuch pebbles. The invention also relates to the use of such pebbles inheat-exchange processes wherein heat is absorbed from a gas in one zoneby a gravitating mass of pebbles and delivered to another gas in asecond zone, with recycling of pebbles and concomitant thermal andmechanical shock to the pebbles.

Pebble heater techniques being developed and applied to various gasheating and reaction processes at the present time make use of a compact stream of small refractory pebbles as a moving heat-exchangemedium. These pebbles which are usually ceramic materials, although theymay be metallic for some applications, are spheres ranging in size fromabout to 1" in diameter. They may be either catalytic or noncatalytic ina given application. In typical pebble heater operation, a continuouscompact mass of pebbles descends by gravity through a series of treatingzones and upon emerging from the lowermost zone, they are elevated by asuitable elevator, usually of the bucket type, to a point above theuppermost zone for again gravitat'in'g' through the system. Theuppermost zone is usually a pebble heating zone where the pebbles arecontacted in countercurrent flow with a stream of hot combustion gas soas to raise the temperature of the pebbles to a desired degree as thepebbles descend through the heating zone. The. heated pebbles then passinto a reaction or gas heating zone where they impart heat to the gasbeing treated and in turn are cooled and require reheating. In someinstallations, a feed gas preheating zone is positioned just below thereaction or gas treating zone so as to further cool the pebbles beforeelevation and to preheat the feed gas to the reaction zone. Otherinstallations utilize a pebble preheating zone positioned directly abovethe pebble heating zone proper where the pebbles are contacted with theefiluent from the reaction zone so as to recover a substantial portionof the sensible heat thereof and simultaneously quench the reactionproduct.

In another type of pebble heat-exchange process, a gravitating' mass ofpebbles is utilized-to mamtain a cold-zone or cool a gas. The pebblesfare cooled by-contactf with" a cold gasin one 2 chamber and the coldpebbles are then gravitated through a second chamber in contact with thegas to be cooled. In such processes the pebbles undergo greatdifferences in temperature with the usual mechanical shock and attritionforces involved in gravitating masses of pebbles.

The pebbles of the invention are utilized to advantage in such processesas those disclosed in my copending applications Serial No. 651,293,filed March 1, 194.6, involving the production of CS2, and Serial No.662,149, filed April 15, 1946, relating to the cracking of hydrocarbonsto hydrogen and coke, as well as the process of the copendingapplication of M. O. Kilpatrick, Serial No. 761,696, filed July 1'7,1947, relating to the thermal conversion of hydrocarbons to moredesirable hydrocarbons. These processes involve temperature changes ofthe order of 1000 to 2000 F. per minute, with severe mechanical shockand abrasive forces present.

The pebble heater finds its greatest utility in operations which requireextremely fast heating rates and therefore extremely fast pebble coolingrates with concomitant thermal shock to the pebbles. In pebble heaterprocesses involving more severe heating and cooling requirements, thepebbles are subjected to heating rates of as much as 1000 F. per minuteand cooling rates of more than 2000 F. per minute at maximumtemperatures in the neighborhood of 3000 F. In addition to the severethermal shock resulting from such rapid temperature changes, the pebblesare subjected to considerable mechanical shock in passing through theapparatus and, especially, in the elevator equipment and in droppingfrom the top of the elevator into the top of the pebble heating zone. Itis found that considerable breakage and loss of pebbles occur when usingconventional commercial pebbles under such severe conditions ofoperation. Pebbles which have been made from powdered alumina, mullite,and similar materials, by wetting the powder and rolling the material inconventional balling equipment until balls of the proper size have beenformed, are found to exhibit laminar structure and suffer breakage underthe strain of pebble heater operating conditions. Pebbles which are madeby slugging and compacting the slugs into spheres do not exhibit thislaminar structure and are much more resistant to breakage under pebbleheater operating conditions. However, it has been found that even whenpebbles have been made by slugging and compacting the slugs "into balls,they must be fired at a temperature within a critical range in order toproperly bond the pebble crystals and produce a pebble which is ruggedunder severe conditions of service. Pure alumina pebbles require firingin the range of 3000 to 3150 F. to develop the most rugged pebble as isdisclosed in my application Serial No. 23,245, filed April 26, 1948; andmy application Serial No. 52,774., filed October 4, 1948, discloses acritical range for firing mullite-bonded and stabilized alumina pebblesbetween 2900 and 3200 F.

It is found that commercially available high purity alumina pebbles arenot satisfactory for moving bed cyclic heat transfer duties for severalreasons. High heating and cooling rates soon cause crystal growth oflarge alpha-corundum crystals at the expense of smaller bondingcrystals. The pebble acquires a granular structure with development oflarge internal cracks. Mechanical shock crumbles these pebbles. Inaddition, major crystal growth occurs with surface crystals wherestrains are greatest. Because of high purity, these crystals are wellformed and exceedingly sharp edged. Attrition losses from individualpebbles wearing upon themselves is excessive as is abrasion of handlingequipment. In addition, non-uniform commercial firing and lack ofappreciation of the value of closely controlled firing within narrowlimits produces large quantities of pebbles poorly attrition resistanteven without heat shock.

In a pebble heater .process requiring the circulation of between 25,000and 35,000 pounds of pebbles per hour with a maximum temperature shockof approximately 1000" F. per minute, the attrition and breakage loss onthe best available commercially produced alumina pebble amounts to atleast 200 pounds per day and runs as high as 700 pounds per day. Thisrepresents a loss of between 0.8 and 2% per day. The alumina pebbleswere selected as the best available commercial pebbles. This substantialloss of pebbles due to attrition and breakage merely emphasizes the needfor a rugged, attrition, and shock resistant pebble. It is with theimprovement of these pebble characteristics that this invention isconcerned.

The invention has several objects, viz.,

To provide an improved alumina pebble having high resistance to breakageunder severe conditions of cyclic thermal and mechanical shock;

To provide a method of heat treating pebbles compacted from alumina andphosphoric acid so as to develop a better bond between crystals andstabilizes the growth of alumina crystals;

To provide a method of manufacturing thermal and mechanicalshock-resistant pebbles free from laminar structure;

To provide an effective method of strengthening the bond between aluminacrystals in a high purity alumina pebble; and

To provide an alumina pebble highly resistant to attrition losses uponitself in continuous moving bed cyclic heat transfer apparatus.

Other objects of the invention will become apparent from a considerationof the accompanying disclosure.

The invention is concerned with a method of manufacturing high purityaluminum phosphatebonded and stabilized pebbles and involvesincorporating a substantially pure alumina mix, P205 between 0.1 and byweight of the alumina as ortho, meta or pyrophosphoric acids or theiranhydrides followed by homogenizing and compacting the mix into pebblesand calcining them at a temperature'in the range of 2800 to 3200 F.,preferably 2950 to 3050 F., for at least two hours 4 and up to 24 hours.The heating in this range should be continued until the porosity of thepebble is in the range of 5 to 25 per cent, and preferably 7 to 15 percent.

The alumina for pebbles is preferably in the form of small alphacorundum crystals and should be at least 99% pure alumina, andpreferably 99.5% alumina. A typical analysis of alumina suitable for theprocess is as follows:

' Percent A1203 99.5 NazO 0.20 F6203 0.25 S102 0.05

The alpha corundum may be made from any aluminum oxide material bysuitable purification and is preferably precalcined at a temperature inthe range of 1800 to 2200 F. for best results. Any of the substantiallypure alumina hydrates which are readily convertible to alpha corundumupon heating to the above range may be used as the source of alumina inthe pebble. Purified bauxite and the alumina manufactured by the Bayerprocess are examples of suitable raw materials for the alumina.

The phosphoric acid to be incorporated with the alumina mix may be inthe ortho-, meta-, or pyro-form or their anhydrides, but the use ofsyrupy phosphoric acid (85% H3PO4) is preferred.

In the preparation of the mix from which the pebbles are to be made, itis highly desirable to comminute the alumina so that it will pass a 200mesh screen and preferably a 325 mesh screen. In order to obtain thebest results, at least of the alumina should be screened or comminutedto pass a 325 mesh screen with the balance of at least 200 meshfineness. The alumina may be powdered before or after the addition ofwater, and the water content of the mix should be adjusted to the rangeof 15 to 25% by weight before the formation of the mix into pebbles. Aslittle as 0.1 per cent of phosphoric acid calculated as P205 based onthe weight of the alumina substantially improves the performance of highpurity alumina pebbles, but to effect desirable bonding and properstabilization of the alpha corundum crystals, up to 10% phosphoric acidmay be incorporated in the mix with desirable results.

In compacting pebbles according to the invention, the alumina mixcontaining between 0.1 and 10% P205 as phosphoric acid is thoroughlyplasticized and homogenized, preferably by mixing in a muller type millfor an extended period and the moisture content is adjusted in the rangeof 15 to 25% in order to provide the proper consistency for extrusion.The stiff paste is then preferably extruded through dies in either apiston or screw type extrusion press into macaroni cylinders or rodswhich are automatically cut off into short lengths (slugs) correspondingto the diameter or cross-section of the rods so as to facilitate theballing step. Drying the paste or mix to a moisture content between 15and 25% is necessary in order to permit proper extrusion of the paste.The moisture content of the alumina paste during the extrusion step isimportant because, when it amounts to less than 15%, the slugs formedfrom the extruded rods are not completely homogeneous in structure andwill result in the formation of an inferior pebble. If the moisturecontent exceeds 25 the extruded rods are too sticky and the slugs cannotbe properly handled in the subsequent balling step. For best performanceduring this step, a moisture content of 17 to 19% by weight is desirabe.When making e" pebbles, extrusion of the plastic mix into rods andcutting into lengths permits the compacting of the slugs intoapproximately unfired pebbles. High pressure extrusion of this type withor without deairing of the feed is much preferred to other methods ofpreparing the slugs for the pebble balling operation to follow, inasmuchas a homogeneous body results, with minimum variations in structureafter firing and avoidance of laminar structure. However, other methodsof preparing the slugs are within the scope of the invention.

Following the cutting of the extruded mix into slugs, the slugs aredried to a moisture content between and by weight before compactin orrolling the slugs into balls, the next step of the operation. Wetterslugs tend to ball up and stick together, while dry slugs roll up intoballs which develop internal cracks upon firing. A preferred moisturecontent for this step lies between 11.5 and 13%. Compacting of thealumina-phosphoric acid slugs into balls or pebbles can be performed inseveral ways. Rolling of the slugs in a balling machine utilizingthree-dimensional rotation in a cylindrical drum placed at angles to allthree axes of conventional rotary equipment is found to make the mostsuitable pebbles upon firing. The balls are more firmly compacted andmore nearly spherical in shape than when made by any other known method.This is probably due to the fact that the slugs are rolled in alldirections during the rolling or compacting step. The resultingspherical pebbles containing the proper amount of moisture do not sticktogether and may be stored temporarily or transferred directly to thefiring step. The balls may be dried to desired moisture content eitherahead of or during the balling operation which should continue for anaverage time of at least 30 minutes. Before high temperature firing, itis necessary to slowly drive off all traces of residual free H content.Rapid drying in the kiln might develop poor'internal structure andcracks if steam were not readily and uniformly removed from the core ofthe pebble. The critical firing temperature of the compacted balls ofalumina-phosphoric acidas stated hereinbefore,

lies in the range'of'2800 to3200" F.

Considerable P205 combines with the more active A1203 as soon as mixingoccurs. The resulting phosphate aids in the plasticizing and initialtemporary bonding of the pebble. The completeness of the reaction willdepend upon the amount of reactive A1203 or Al(OH)3 present in in theinitial mix. If none is present, additional plasticizing agents oforganic or inorganic nature must be added. As the firing temperatureincreases, the P205 is substantially converted to AlP04, and residualunreacted A1202 is slowly converted to alpha corundum. When crystaldevelopment of such, normally becomes rapid around 2800-2900 F., theAlPOi begins to soften or melt to a very viscous high temperature glasswhich effectively coats and separates major portions of the unreactedalumina and eifectively stabilizes alpha corundum crystal growth. Atlower operating temperatures, A1PO4 is a very strong and effectivebonding agent for A1203 crys tals. The temperature and length of firingordinarily determine the porosity of the pebble, but when an organicbinding material is incorporated in the pebble mix to aid in the pebbleballing operation, the amount of organic binding material incorporatedtherein determines to some extent the porosity of the pebble. The firingshould be continued in the range of 2800 to 3200" F. at least two hoursand until the porosity of the pebble lies in the range of 7 to 15%. Theterm porosity is intended to include both the connected and sealed offpore space. When properly fired 1% pebbles have a crushing strengthexceeding 1000 pounds applied to parallel plates.

Firing or calcination of the pebbles can be suitably effected in anyconventional equipment which results in maintaining the entire mass ofpebbles at a relatively even temperature within the specified range.Batch firing in continuous shaft kilns produces pebbles which areinferior in service in pebble heater operation because they are notuniformly heated in all parts of the bed, a large proportion beingeither underfired or overfired. The former are not strong and stand uppoorly to both heat and mechanical shock, while the latter are too rigidand soon develop cracks along large crystal faces which results in earlybreakage in service. Neither are attrition resistant.

The following examples illustrate two specific modifications of theinvention and are not to be construed as unduly limiting the invention:

Example I 47 lbs. of H3P04 are added to 900 lbs. of Bayer processalumina precalcined to 2100 F. and 60 lbs. of purified but active lightcalcined A: and enough water to produce a stiff paste of 16% H20 contentafter intensive mixing in a mulling pan mixer. This material is extrudedwithout deairing in a piston type extrusion press equipped with dies andautomatic knives to prepare diameter by 7 long slugs. These are tumbledinto spheres in a three dimensionally rotated tumbling drum swept withflue gas to produce balls with 12% moisture content after 30 minutestumbling. These are dried in a waste heat or Dutch oven type dryerbefore calcining for 12 hours at 3000 F. in a tunnel kiln. A pebble offinal composition of approximately 5% AlP04-95% A1203 is produced havingvery desirable heat and mechanical shock and attrition resistantproperties. Porosity is reduced to 10 to 12%; alphacorundum crystalsaverage under 25 microns in size; and crushing strength in diameterpebbles applied between parallel plates exceeds 1500 lbs. Surfacecrystals are small and essentially bedded in AlP04 material.

Example II 30 lbs. of P205, 900 lbs. of precalcined (1800- 2200 F.)Bayer process alumina and 70 lbs, of light reactive alumina are mixedwith enough water to produce a stiff paste of 18% H20 content. Paste isprocessed as in Example I after extrusion in an auger type deairingextrusion press. Final product is burned in a periodic kiln 4 hours at3100 F. Essentially the same type of pebble is produced as that ofExample I.

Certain modifications of the invention will become apparent to thoseskilled in the art and the illustrative details disclosed are not to beconstrued as imposing unnecessary limitations on the invention.

I claim:

1. A process for manufacturing stabilized alumina contact material inthe form of pebbles capable of withstanding cyclic thermal andmechanical shock over long Periods without breakas es-e1 age and highlyattrition resistant, which comprises compacting /8" to 1" spheres fromfinely comminuted active alumina and phosphoric acid in which thephosphoric acid (calculated as P205) amounts to between 0.1 and 10% byweight of the alumina; slowly drying said spheres; and calcining thedried spheres at a temperature in the range of 2800 to. 3200 F. for atleast 2 hours and until the porosity lies. in the range of 5 to 25%.

2'. A process for manufacturin stabilized alumina contact material inthe form of pebbles capable of withstanding cyclic thermal andmechanical shock over long periods without breakage and highly attritionresistant, which comprises forming a homogeneous plastic mix of finelyeomminutedactive alumina of at least 99 purity and phosphoric acidsuitable for extrusion into rods, the phosphoric acid (calculated asP205) amounting to between 0.1 and by weight of the alumina; extrudingsaid mix into rods A3" to 1" in diameter; cutting said rods into slugsbetween /8" and 1" in length; compacting the slugs into balls; andcalcining said balls at a temperature in the range of 2800 to 3200 F.for at least 2 hours and until the porosity lies in the range of '7 to3. The process of claim 2 in which, the balls are calcined in the rangeof 2950 to 3050 F.

4. A process for manufacturing stabilized alumina contact material inthe form of pebbles capable of withstanding cyclic thermal andmechanical shock over long periods without breakage, which comprisesforming a homogeneous aqueous plastic mix of finely comminuted activealumina, phosphoric acid, and water in which the phosphoric acid.(calculated at P205) amounts to between, 0.1 and 10% by Weight of thealumina; adjusting the water content to the range of 15 to 25% by Weightof the mix; forming the mix into slugs suitable for comp-acting in A3"to 1" balls; drying said slugs to a water content in the range of 10 to15% by weight; compacting the partially dried slugs into balls; andcalcining the balls at a temperature in the range of 2800 to 3200 F. forat least 2 hours and until the porosity lies in the rangeof 7 to. 15%.

'5. The process of claim 4 in which an organic binder is incorporated inthe mix in an amount between 2 and 10% by weight thereof.

6. A process for manufacturing stabilized alumina contact material inthe form of pebbles capable of withstanding cyclic thermal andmechanical shock over long periods without breakage, which comprisesforming a homogeneous, aqueous, plastic mix of finely divided activealumina at least 80% of which passes a 325 mesh screen, phosphoric acid,and. water, in which the acid (calculated as P205) amounts to between0.1 and,10% by weight of the alumina; adjusting the. water to the rangeof 15 to 25 by weight of the mix; extruding the mix into A, to 1" rods;dividing the rods into slugs /8 to 1" in length; drying the slugs to awater content in the range of. 10 to 15% by Weight; compacting thepartially dried slugs into balls by rolling and tumbling; slowly dryingthe balls to a water content less than 1%; and calcining the dried ballsat a temperature of 2950 to 3050 F. for at least 2 hours and until theporosity lies in the range of 7 to 15% thereby forming A1PO4 in situ andstabilizing growth of alphaalumina crystals in the balls up to thecalcining temperature.

'7. The process of claim 6 in which avolatile organic binder isincorporated in the mix in an amount between 2 .and 10% by weightthereof.

8. A method of heat treating pebbles compacted from a moist mix ofalumina and phosphonic acid so as to improve the attrition and breakageresistance thereof under severe conditions of cyclic thermal andmechanical shock, which comprises slowly drying said. pebbles andcalcining the dried pebbles at a temperature in the range of. 2800 to3200 F. for at least 2 hours and until the porosity thereof lies in therange of 7 to 15%, thereby forming AlPO4 in situ and stabilizing thealumina crystals against further growth at temperatures up to thecalcining temperatune.

9. A stabilized alumina pebble produced by the process of claim 1 andconsisting essentially of to 99.8 weight per cent alumina, 0.2 to 19weight per cent aluminum phosphate, and not over 1 weight per cent othermaterials, the aluminum phosphate being dispersed uniformly throughoutthe pebble so as to form a substantially homogeneous composition, thealumina crystals being of an average size of less than 25 microns, saidpebble having a crushing strength of at least 1000 pounds (based on a1%" pebble),v

SAM P. ROBINSON.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,949,038 Caven Feb. 27, 19342,007,742 Brown July 9, 1935 2,035,845 Stanton Mar. 31, 1936 2,061,099Morgan Nov; 17, 1936 2,304,133 Wilson et a1. Dec. 8, 1942 2,389,636Ramseyer Nov. 27, 1945 2,463,979 Langrod Mar. 8, 1949

1. A PROCESS FOR MANUFACTURING STABILIZED ALUMINA CONTACT MATERIAL INTHE FORM OF PEBBLES CAPABLE OF WITHSTANDING CYCLIC THERMAL ANDMECHANICAL SHOCK OVER LONG PERIODS WIHTOUT BREAKAGE AND HIGHLY ATTRITIONRESISTANT, WHICH COMPRISES COMPACTING 1/8" TO 1" SPHERES FROM FINELYCOMMINUTED ACTIVE ALUMINA AND PHOSPHORIC ACID IN WHICH THE PHOSPHORICACID (CALCULATED AS P205) AMOUNTS TO BETWEEN 0.1 AND 10% BY WEIGHT OFTHE ALUMINA; SLOWLY DRYING SAID SPHERES; AND CALCINING THE DRIED SPHERESAT A TEMPERATURE IN THE RANGE OF 2800* TO 3200* F. FOR AT LEAST 2 HOURSAND UNTIL THE POROSITY LIES IN THE RANGES OF 5 TO 25%.